The use of electronic trip units in low-ampere industrial-rated circuit breakers has often been forestalled because of size constraints on the discreet electrical and electromagnetic components within the circuit breaker enclosure. The current transformer used in conjunction with the electronic trip unit, has a two-fold requirement namely, to provide an input signal to the trip unit representative of the current flow within the associated protected power circuit while providing the necessary input power to the trip unit power supply. A predetermined maximum core volume is required within the current transformer to ensure that the current transformer does not become magnetically saturated upon the occurrence of overcurrent conditions when used within compact circuit breakers having variable ampere ratings while a predetermined minimum core volume insures that the core will become sufficiently magnetized at the lower steady-state operating current levels.
With earlier-designed electronic trip circuit breakers, such as described within U.S. Pat. No. 4,281,359 entitled "Static Trip Unit for Molded Case Circuit Breakers", for example, a standard trip unit circuit is employed over a wide range of ampere ratings while the size of the current transformer used to sense the input current to the trip unit circuit is correspondingly increased in proportion to the increased ampere rating.
When compact electronic trip unit circuit breakers employing various accessory devices, such as described in U.S. Pat. No. 4,754,247 entitled "Molded Case Circuit Breaker Accessory Enclosure", are used within industrial rated power distribution circuits, the size constraints of the circuit breaker enclosure limit the geometry of the current transformer core to a size just sufficient to provide operating power to the electronic trip unit circuit without becoming saturated at the higher ampere ratings due to the low inductance of the smaller core. Another problem involved with the use of small-sized current transformer cores is the lack of sufficient core inductance to provide the requisite core magnetization for transformer operation at the lower ampere ratings.
U.S. Pat. No. 5,515,597 entitled "Method for Assembling a Current Transformer" describes a compact core arrangement for current transformers and the like that is accomplished by winding the secondary coils around the completed magnetic core.
U.S. Pat. No. 5,015,983 entitled "Compact Circuit Interrupter Having Multiple Ampere Ratings" describes a compact current transformer arrangement using a metal core formed from laminations of silicon sheet steel positioned over a pair of secondary windings. The air gaps inherent with such laminated steel plates increase the core losses that are subsequently compensated for by increasing the core size and the amount of core material.
U.S. patent application Ser. No. 08/663,760 entitled "Compact Circuit Interrupter Having Multiple Ampere Ratings" describes a compact current transformer used within circuit breakers having electronic trip units. The continuous arrangement of a sheet of core material about the primary winding provides a transformer core without air gaps. When current transformers are formed about a pair of secondary winding coils the two coils are separately wound on individual bobbins and are later electrically connected together such that the directions of the wires in each of the coils is in opposite directions. One early example of a pair of miniature coil bobbins used within telephone receivers is found within U.S. Pat. No. 4,103,268 entitled "Dual Coil Hinged Bobbin Assembly".
It would be advantageous to arrange the separate bobbins in such a manner that the coils could then be wound from a continuous source of wire without requiring separate connection operations and orientation as is now required.
One purpose of the invention is to provide a unique bobbin arrangement that allows the two coils to be wound from a single source of wire to eliminate the inter-coil connection process.